In fiber-optic sensors, it is often advantageous to conduct the radiation traveling to the sensor head and the radiation returning from the sensor head through one and the same fiber. This corresponds to the duplex method (bidirectional transmission) known from communications technology wherein information is transmitted through a fiber in both directions.
An assembly suitable for fiber-optic sensors for coupling the radiation of a radiation source into a fiber and for conducting the returning radiation to a receiver is disclosed, for example, in published German patent application DE-OS 3,431,997. A suitable arrangement having a reference receiver is described in U.S. Pat. No. 4,707,838 for the control of the radiation emitted by a light-emitting diode and for its control. Such arrangements utilize a conventional optical assembly having normal lenses and dichroic layers on plane-parallel plates for deflecting beams in dependence upon wavelength. These arrangements have the disadvantage that they require a complex assembly because of the necessary mounting of their individual elements. In addition, the adjustment of these arrangements is complicated and they are poorly suited for miniaturization. This applies especially for more complex arrangements, for example, when a returning beam and intermittently a reference beam are to impinge on the same receiver.
An optical switch for transmitting information in two directions through a fiber in the so-called wavelength multiplex method is known from German Patent 3,232,445. This optical switch comprises a plurality of mutually abutting graded index rod lenses (GRIN-lenses) and filters and the receiver is mounted at one end thereof. On the opposite-lying end face of the other end, a fiber for information transmission is connected as well as a fiber which leads to a semiconductor transmitter. The filters operate as reflection filters for the radiation of the semiconductor sensor so that the radiation reaches the fibers for transmitting information. The incoming radiation of the information transmission has, however, a wavelength which the filter passes so that the radiation arrives at the receiver.
This solution provided for the transmission of information has the disadvantage that the fibers do not seat coaxially on the end face of the first GRIN-lens. This not only means a greater production cost but also a compromise between a greater cross coupling between the fibers (with decreasing spacing of the fibers) and deteriorating imaging characteristics of the GRIN-lenses (with increasing spacing of the fibers) and greater losses caused thereby. This known solution is therefore not suitable for fiber-optic sensors wherein the returning radiation is considerably less than the radiation traveling to the sensor head.
A multiplexer is known from an article of W. J. Tomlinson entitled "Applications of GRIN-rod lenses in optical fiber communication systems", Applied Optics, Vol. 19, No. 7, Apr. 1, 1980 wherein the radiation supplied via a row of fibers is conducted with different wavelengths into a common fiber. The fibers are coupled to a plane-parallel plate via , GRIN-lenses and prisms and suitable filters are mounted on both surfaces of the plane-parallel plate. The optical axes of the beams define equal acute angles to the normal on the plate surfaces and, with each reflection at a filter, the radiation is joined with the radiation admitted by this filter into the plane-parallel plate.
If this arrangement is reduced to two wavelengths, then an acute angle switch having a filter results which passes the one wavelength and reflects the other wavelength. An optical switch of this kind would be suitable for a fiber-optic sensor but has the disadvantage that it is not suitable for sensors wherein a reference beam path is necessary with which the radiation of the radiation source can be intermittently conducted directly onto the receiver.